M. Gregor

Venezuelan sandstone caves: a new view on their genesis, hydrogeology and speleothems

Caves in arenites of the Roraima Group in Venezuela have been explored on the Chimanta and Roraima plateaus (tepuis). Geological and geomorphological research showed that the most feasible method of caves genesis was the winnowing and erosion of unlithified or poorly lithified arenites. The unlithified arenitic beds were isolated by well-cemented overlying and underlying rocks. There is a sharp contrast between these well-lithified rocks and the loose sands which form the poorly lithified to unlithified beds. They are only penetrated by strongly lithified pillars which were cemented by vertical finger flow of the diagenetic fluids from the overlying beds. Such finger flow is only typical for loose sands and soils where there is a sharp difference in hydraulic conductivity. The pillars exhibit no signs of further dissolution. The caves form when the flowing water accesses the poorly lithified beds through clefts/crevices. Collapse of several superimposed winnowed-horizons can create huge subterranean spaces. Futher upward propagation of the collapses can lead to large collapse zones which are commonly observed on the tepuis. Dissolution is also present but it probably plays neither a trigger role, nor a volumetrically important role in the cave-forming processes. The strongest dissolution/reprecipitation agent is condensed atmospheric moisture which is most likely the main agent contributing to growth of siliceous speleothems. As such, it can be active only after, but not before the cave is created. Siliceous speleothems are mostly microbialites except for some normal stalactites, cobweb stalactites and flowstones which are formed inorganically. They consist of two main types: 1. fine-laminated columnar stromatolite formed by silicified filamentous microbes (either heterotrophic filamentous bacteria or cyanobacteria) and 2. a porous peloidal stromatolite formed by Nostoc-type cyanobacteria. The initial stages of encrusted shrubs and mats of microbes were observed, too, but the surrounding arenitic substrate was intact. This is strong evidence for the microbial mediation of silica precipitation.

Effect of crystallographic anisotropy on the resistance switching phenomenon in perovskites

Resistance switching effects in metal/perovskite contacts based on epitaxial c-axis oriented YBa2Cu3O6+c (YBCO) thin films with different crystallographic orientation have been studied. Three types of Ag/YBCO junctions with the contact restricted to (i) c-axis direction, (ii) ab-plane direction, and (iii) both were designed and fabricated, and their current-voltage characteristics have been measured. The type (i) junctions exhibited conventional bipolar resistance switching behavior, whereas in other two types the low-resistance state was unsteady and their resistance quickly relaxed to the initial high-resistance state. Physical mechanism based on the oxygen diffusion scenario, explaining such behavior, is discussed.

Surface transport properties of Fe-based superconductors: The influence of degradation and inhomogeneity

Surface properties of Co-doped BaFe2As2 epitaxial superconducting thin films were inspected by X-ray photoelectron spectroscopy, scanning spreading resistance microscopy (SSRM), and point contact spectroscopy (PCS). It has been shown that surface of Fe-based superconductors degrades rapidly if being exposed to air, what results in suppression of gap-like structure on PCS spectra. Moreover, SSRM measurements revealed inhomogeneous surface conductivity, what is consistent with strong dependence of PCS spectra on contact position. Presented results suggest that fresh surface and small probing area should be assured for surface sensitive measurements like PCS to obtain intrinsic properties of Fe-based superconductors.

Sorption of P(V), As(V), and Sb(V) Oxyanions on Goethite and Hematite During their Thermal Transformation

Sorption of pentavalent oxyanions P(V), As(V), and Sb(V) was studied on goethite and hematite prepared by its thermal transformation. The surface properties of goethite and products of its thermal modification at different temperatures were studied by BET method, FT-IR, XRD, and DTA-TGA. Amounts of immobilized ions reached their maxima on sorbent prepared at 250°C. Changes of the specific surface area (32.5 m2.g−1 at 150°C, 82.3 m2.g−1 at 250°C and 34.8 m2.g−1 at 350°C) during the thermal transformation at different temperatures were observed. Further analysis confirmed the complete transformation of goethite to hematite at temperatures 200 − 250°C accompanied with the disappearance of hydroxyl absorption bands at ∼800 and ∼900 cm−1 in FT-IR spectrum and significant loss of weight observed on TGA curve. The study of adsorption isoterms revealed that antimony has higher affinity for all studied sorbents.

Analysis of Hydrological Drought

This chapter, entitled Analysis of hydrological drought is the first part of a self author’s work. It deals with a comprehensive analysis of hydrological drought. This chapter is divided into several subchapters. The first deals with the analysis of hydrological drought in the rivers profiles in Slovakia. This chapter includes profiles that are distributed through the entire Slovakia and these profiles were assessed for determining the basic characteristics and regularities of the drought occurrences and their time course in the regional scale, especially as a reference source for local assessment of drought in the upper part of the Nitra river catchment.

Recommendation for Further Research

Many new questions have arisen from this thesis and its results, and answers will require further work and a lot of effort. In assessing the meteorological drought, the regionalization of results obtained from observed points in individual meteorological stations was quite interesting. The results from all the individual stations were averaged for this entire evaluated area which involved considerable simplification. Spatial distribution of the meteorological stations and errors measurement can significantly distort overall results. Therefore, it is appropriate to study the regionalization methods, which can bring new knowledge.

Water Quality Assessments in Dry Seasons

This chapter focuses on the water quality assessment in dry seasons, and it is divided into several subchapters. The first subchapter is devoted to the input data and evaluation methods for surface and groundwater quality assessment. The following three subchapters deal with general, spatial and temporal assessment of surface and groundwater quality in the upper part of the Nitra river catchment. The next part concerns water quality evaluation in the assessed area in the differing surface and groundwater quantitative states. The last subchapter briefly summarizes the knowledge acquired from this water quality evaluation.

Summary of Thesis Results

The first result of this author’s contribution is marked by development of new hydrological and hydro-geochemical software tools. Their ongoing gradual development has exceeded their initial focus on hydrological drought assessment and the evaluation of water quality. They currently present a comprehensive, multi-purpose package which begins with the processing of various time series of measurement and ends with modelling infiltration processes in catchments, and solute transport modelling in dynamic rives systems.

Software Tools Used at Work

At the processing of the presented thesis we used larger number of software tools. The essential tool was table calculator—MS Excel. From the additional tools there was used for spatial analysis the open-source tool MapWindow 6, allowing using number of spatial functions as well as framework for spatial analysis. As often happens, software tools are not sufficient, especially for specific purposes and goals. Therefore, author of this thesis designed and programmed several new programs

Area of Interest and its Natural Conditions

Area of interest, where we have processed the majority of analyses in present work is located in the western part of Slovakia in the territory of upper part of the Nitra river catchment (Fig. 4.1).